This invention relates to a calender for the thermal treatment of laminar material, and in particular to a calender intended for the thermo-printing of a pressure-sensitive textile material.
A thermo-printing process consists of bringing the material to be printed into contact, under pressure, with an inert support carrying a design which is to be transferred. This design is executed in dyes which can be vapourised or sublimated. The whole assembly is then brought to a temperature suitable for causing evaporation or sublimation of the dyes and for making them pass to the material to be printed, during a time which is determined as a function of the intensity of the shade to be obtained. The treatment temperature is generally of the order of 220.degree. C and the contact duration of the order of 20 seconds tp 5 minutes.
This treatment, when it is carried out continuously on materials of great length, is put into effect in a calender constituted by a heated cylindrical drum which is rotatable about its axis and against which the material to be printed and the inert support for the design to be transferred, now offered up in the form of a band, are applied one onto the other by an endless longitudinal belt or "messenger", the linear speed of which is equal to the circumferential speed of the heated drum, which it partially hugs around the cylindrical periphery.
Depending upon the nature of the fibres which make up the material to be thermo-printed and according to the nature of the dyes used, the belt and the "messenger" may be of different natures, in particular they may be impermeable or otherwise, and the calender may include various auxiliary devices for the purpose notably of facilitating thorough penetration of the dyes in a direction perpendicular to the general plane of the material printed, while at the same time avoiding diffusion along this plane, which would result in a blurred or inaccurate reproduction of the contours of the design.
None of the arrangements at present known give satisfaction, however, when it is desired to thermoprint textile materials which are sensitive to pressure, for example carpets, notably when these materials are of great length.
In fact, the quality of the thermo-printing obtained on such materials is very much influenced by the contact pressure of the inert support for the dyes and of the material to be thermo-printed and, even though it is possible to regulate this pressure to a certain extent by a greater or lesser tension of the messenger by means of a roll having a transverse axis applied onto one of its faces, no device at present known enables a uniform pressure to be obtained across the entire width of the belt or messenger, especially if this width is as large as, for example, 5 meters, as is the case when carpets are being printed.
In fact, differences in pressure of the order of 20 g per cm.sup.2 are encountered between different zones distributed across the width of the messenger, these pressure differences being caused in particular by the distortions to which the messenger is inevitably subjected in its translatory movement along its own direction. Such differences in pressure in the direction of the width are not very harmful when, for example, woven fabrics are being treated, the required pressure then being generally of the order of 100 to 150 g per cm.sup.2, but they have a very adverse effect when materials which require a low working pressure are being handled. For example, a pressure of the order of 5 to 50 g per cm.sup.2 is required in the case of carpets, as carpets must be treated at a pressure which is both sufficient to ensure good penetration of the dyes in depth and to prevent their diffusion along the general plane of the carpet, and which at the same time is sufficiently low for the carpet, or more generally the material treated, not to adopt a crushed appearance. In the case of these low pressures, the differences of pressure recorded between the different zones of the messenger lead to noticeable differences in the appearance of the thermo-printed material.
The aim of the present invention is to overcome this disadvantage, and to achieve this result without modifying the nature of the messenger itself, that is to say in practice the nature of the materials of which the messenger is made and the way they are presented, which are generally dictated by the conditions of thermo-printing adapted notably to the fibres treated.